This invention relates to optical measurements and especially to the measurement of the distance between two light-reflecting surfaces by means of a single laser pulse.
Various ways of measuring the separation between two optical surfaces are known. For example, there is a pulsed method using ultra-short pulses of light. If the arrival times of the reflections from the surfaces can be measured with sufficient accuracy, then the surface separation can be found. While picosecond pulses can be generated, timing circuitry limits the resolution of surface separations to the order of a centimeter. The resolution obtainable with the present invention is on the order of 0.4 cm. Higher resolution can be obtained by reducing the coherence length of the pulsed laser source.
The use of a pulsed light source in the measurement of surface separation is of advantage. It enables moving targets to be interrogated; it minimizes problems due to vibrations; and since both reflected light beams return over the same optical path, the technique is suitable for use through the atmosphere provided sufficient energy to expose the film is available.